1 /* Compute a product of X, X+1, ..., with an error estimate.
2 Copyright (C) 2013-2022 Free Software Foundation, Inc.
3 This file is part of the GNU C Library.
4
5 The GNU C Library is free software; you can redistribute it and/or
6 modify it under the terms of the GNU Lesser General Public
7 License as published by the Free Software Foundation; either
8 version 2.1 of the License, or (at your option) any later version.
9
10 The GNU C Library is distributed in the hope that it will be useful,
11 but WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
13 Lesser General Public License for more details.
14
15 You should have received a copy of the GNU Lesser General Public
16 License along with the GNU C Library; if not, see
17 <https://www.gnu.org/licenses/>. */
18
19 #include <math.h>
20 #include <math-narrow-eval.h>
21 #include <math_private.h>
22 #include <float.h>
23
24 /* Compute the product of X + X_EPS, X + X_EPS + 1, ..., X + X_EPS + N
25 - 1, in the form R * (1 + *EPS) where the return value R is an
26 approximation to the product and *EPS is set to indicate the
27 approximate error in the return value. X is such that all the
28 values X + 1, ..., X + N - 1 are exactly representable, and X_EPS /
29 X is small enough that factors quadratic in it can be
30 neglected. */
31
32 double
__gamma_product(double x,double x_eps,int n,double * eps)33 __gamma_product (double x, double x_eps, int n, double *eps)
34 {
35 long double x_full = (long double) x + (long double) x_eps;
36 long double ret = x_full;
37 for (int i = 1; i < n; i++)
38 ret *= x_full + i;
39
40 double fret = math_narrow_eval ((double) ret);
41 *eps = (ret - fret) / fret;
42
43 return fret;
44 }
45